1. Field of the Invention
The present invention relates to a fabrication method of a microstructure, a fabrication method of a mold or a master of a mold (in the specification the term “mold” is chiefly used in a broad sense including both a mold and a master of a mold) for forming a microstructure, a fabrication method of a microstructure using the mold, and so forth. This invention particularly relates to a mold for a microlens or a microlens array, a fabrication method of the mold, and a fabrication method of the microlens or the microlens array using the mold.
2. Description of the Related Background Art
A microlens away typically has a structure of arrayed minute lenses each having a diameter from about 2 to 3 microns to about 200 or 300 microns and an approximately semispherical profile. The microlens away is usable in a variety of applications, such as liquid-crystal display devices, light receivers and inter-fiber connections in optical communication systems.
Meanwhile, earnest developments have been made with respect to a surface emitting laser and the like which can be readily arranged in an array form at narrow pitches between the devices. Accordingly, there exists a significant need for a microlens array with narrow lens intervals and a large numerical aperture (NA).
Likewise, a light receiving device, such as a charge coupled device (CCD), has been more and more downsized as semiconductor processing techniques develop and advance. Therefore, also in this field, the need for a microlens array with narrow lens intervals and a large NA is increasing.
In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency which can highly efficiently utilize light incident on its lens surface.
Further, similar desires exist in prospective fields of optical information processing, such as optical parallel processing-operations and optical interconnections. Furthermore, display devices of active or self-radiating types, such as electroluminescent (EL) panels, have been enthusiastically studied and developed, and a highly-defined and highly-luminous display has been thus proposed. In such a display, there is a heightened desire for a microlens array which can be produced at a relatively low cost and with a large area as well as with a small lens size and a large NA.
There are presently a number of prior art methods for fabricating microlenses.
In a prior art microlens-array fabrication method using an ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20(1) L51-54, 1981), a refractive index is raised at plural places in a substrate of multi-component glass by using an ion exchange method. A plurality of lenses are thus formed at high-refractive index places. In this method, however, the lens diameter cannot be large, compared with intervals between lenses. Hence, it is difficult to design a lens with a large NA. Further, the fabrication of a large-area microlens array is not easy since a large scale manufacturing apparatus, such as an ion diffusion apparatus, is required to produce such a microlens array. Moreover, an ion exchange process is needed for each glass, in contrast with a molding method using a mold. Therefore, variations of lens quality, such as a focal length, are likely to increase between lots unless the management of fabrication conditions in the manufacturing apparatus is carefully conducted. In addition to the above, the cost of this method is relatively high, as compared with the method using a mold.
Further, in the ion exchange method, alkaline ions for ion-exchange are indispensable in a glass substrate, and therefore, the material of the substrate is limited to alkaline glass. The alkaline glass is, however, unfit for a semiconductor-based device which needs to be free of alkaline ions. Furthermore, since a thermal expansion coefficient of the glass substrate greatly differs from that of a substrate of a light radiating or receiving device, misalignment between the microlens array and the devices is likely to occur due to a misfit between their thermal expansion coefficients as an integration density of the devices increases.
Moreover, a compressive strain inherently remains on the glass surface which is processed by the ion exchange method. Accordingly, the glass tends to warp, and hence, a difficulty in joining or bonding between the glass and the light radiating or receiving device increases as the size of the microlens array increases.
In another prior art microlens-array fabrication method using a resist reflow (or melting) method (see D. Daly, et al., Proc. Microlens Arrays Teddington., p 23-34, 1991), resin formed on a substrate is cylindrically patterned using a photolithography process and a microlens array is fabricated by heating and reflowing the resin. Lenses having various shapes can be fabricated at a low cost by this resist reflow method. Further, this method has no problems of thermal expansion coefficient, warp and so forth, in contrast with the ion exchange method.
In the resist reflow method, however, the profile of the microlens is strongly dependent on the thickness of resin, wetting condition between the substrate and resin, and heating temperature. Therefore, variations between lots are likely to occur while a fabrication reproducibility per a single substrate surface is high.
Further, when adjacent lenses are brought into contact with each other due to the reflow, a desired lens profile cannot be secured due to the surface tension. Accordingly, it is difficult to achieve a high light-condensing efficiency by bringing the adjacent lenses into contact and decreasing an unused area between the lenses. Furthermore, when a lens diameter from about 20 or 30 microns to about 200 or 300 microns is desired, the thickness of deposited resin must be large enough to obtain a spherical surface by the reflow. It is, however, difficult to uniformly and thickly deposit the resin material having desired optical characteristics (such as refractive index and optical transmissivity). Thus, it is difficult to produce a microlens with a large curvature and a relatively large diameter.
In another prior art method, an original plate of a microlens is fabricated, lens material is deposited on the original plate and the deposited lens material is then separated. The original plate or mold is fabricated by an electron-beam lithography method (see Japanese Patent Application Laid-Open No. 1 (1989)-261601), or a wet etching method (see Japanese Patent Application Laid-Open No. 5 (1993)-303009). In these methods, the microlens can be reproduced by molding, variations between lots are unlikely to occur, and the microlens can be fabricated at a low cost. Further, the problems of alignment error and warp due to the difference in the thermal expansion coefficient can be solved, in contrast with the ion exchange method.
In the electron-beam lithography method, however, an electron-beam lithographic apparatus is expensive and a large investment in equipment is needed. Further, it is difficult to fabricate a mold having a large area more than 100 cm2 (10 cm-square) because the electron beam impact area is limited.
Further, in the wet etching method, since an isotropic etching using a chemical action is principally employed, an etching of the metal plate into a desired profile cannot be achieved if composition and crystalline structure of the metal plate vary even slightly. In addition, etching will continue unless the plate is washed immediately after a desired shape is obtained. When a minute microlens is to be formed, a deviation of the shape from a desired one is possible due to an etching lasting during a period from the time a desired profile is reached to time the microlens is reached.
Further, there also exists a mold fabrication method using an electroplating technique (see Japanese Patent Application Laid-Open No. 6 (1994)-27302). In this method, an insulating film having a conductive layer formed on one surface thereof and an opening is used, the electroplating is performed with the conductive layer acting as a cathode, and a protruding portion acting as a mother mold for a lens is formed on a surface of the insulating film. The process of fabricating the mold by this method is simple, and cost is reduced. Similar such methods are also disclosed in Japanese Patent Application Laid-Open No. 8 (1996)-258051 and Japanese Patent Publication No. 64 (1989)-10169.
The problem occurring when a plated layer is formed in an opening by the electroplating technique will be described by reference to FIGS. 1A-1D and 2. FIGS. 1A-1D illustrate electroplating growth steps at the time when the electroplating is performed on an insulating body 202 with an opening 203 having a diameter φ and a cathode of an electrically-conductive body 201. FIG. 2 illustrates a cross-section of a mold shown in FIG. 1D.
During the electroplating growth process, a flat portion 210 of a plated layer 204 in the opening 203 gradually decreases as the plated layer 204 grows (FIGS. 1B and 1C), and a surface of the plated layer 204 comes to have a maximum curvature (FIG. 1D). After that, a bottom diameter Ψ of the plated layer 204 increases as the plated layer 204 grows. A radius of curvature thus increases accordingly.
Where the plated layer 204 has the flat portion as illustrated in FIGS. 1B and 1C, the illustrated structure cannot be used as a mold for a lens since image-forming cannot be achieved at a lens portion corresponding to the flat portion. According to this method, a radius of curvature near an optical axis of the lens mold increases as the diameter φ of the opening 203 increases. Where the lens mold is to be fabricated, the radius of curvature of the plated layer 204 cannot be decreased when the diameter φ of the opening is large. Thus, it is often difficult to fabricate a lens mold having a desired radius R of curvature.
Further, where a profile of the plated layer deviates from a semispherical shape as illustrated in FIG. 2, a radius S of curvature of a side surface of the plated layer differs from that R of a portion of the lens near its optical axis and positions of their curvature centers C and C′ differ from each other. Thus, the curvature center C′ of the side surface does not lie on the optical axis. As a result, a side surface of a microlens fabricated using such a mold cannot be used, and hence, the lens will inevitably have a small NA.
Furthermore, it is apparently difficult to precisely form a microlens with a diameter less than several hundreds of microns (particularly, less than about three hundred microns) and a radius of curvature less than 200 μm at an uppermost spherical portion, by the prior art lens fabrication techniques.